It has been generally well recognized that it is necessary to kill or inactivate xx oocysts so that they do not infect. This is especially important in drinking water. One such method is the use of ultraviolet ("UV") light. The prior art teaches that a UV dose of at least 3000 mJ/cm.sup.2 is required to inactivate Cryptosporidium parvum (Lorenzo-Lorenzo et al., J. Parasitol. 1993, 79, 67-70) and Giardia muris (E. L. Jarol, "Effect of disinfectants on Giardia cysts", CRC Critical Reviews in Environmental Control, 1988, 18, 1-28.). Snowball and coworkers (UK Patent Application #9416287.2, Nov. 8, 1984; Wat. Res., 1995, 29, 2583-2586) developed an apparatus that first filtered out Cryptosporidium oocysts and then exposed them to UV doses of 350-400 mJ/cm.sup.2. The patent teaches the use of membrane filters to trap Crysptosporidium oocysts which is then irradiated with a bank of low pressure Hg lamps for a UV dose of 350-400 mJ/cm.sup.2. The filter is then backwashed onto a second filter and the irradiation is repeated. The patent discloses that the treatment "kills" the organisms.
M. J. Lorenzo-Lorenzo, M. E. Area-Mazea, I. Villacorta-Martinez de Maturana and D. Duran-Oreiro, "Effect of Ultraviolet Disinfection of Drinking Water on the Viability of Cryptosporidium parvum Oocysts", J Parasitol. 1993, 79(1), 67-70.The paper reports the prevention of infection in mice after exposure to at least 150 min of UV from a (presumably) low-pressure Hg lamp. Although the paper is not clear, it can be inferred that the UV dose applied was over 5000 mJ/cm.sup.2 to obtain better than 2 logs reduction in infectivity. The authors claim that exposure to UV for 150 min or more "eliminates" infectivity, but they give no mechanism other than to say "UV radiation disrupts DNA by causing formation of th[y]amine dimers, and high levels may leads to cell death. At the UV doses they applied, almost certainly the effects observed were caused by cell death.
In a paper by A. Bushnell, W. Clark, J. Dunn and K. Salisbury, "Pulsed Light Sterilization of Products Packaged by Blow-Fill-Seal Techniques", Pharm. Engin. 1997, September/October, 74-83, a pulsed UV technique for "sterilizing" surfaces containing bacteria, fungi, spores, viruses, protozoa and oocysts is described. The required UV doses were reported to be over 1000 mJ/cm.sup.2. The effectiveness of the method was assayed using mouse infectivity. At the reported UV doses, the effects were believed to be due to cell death.
In a paper by R. LaFrenz, "High Intensity Pulsed UV for Drinking Water Treatment", Proc. AWWA WQT Conference, Denver, Colo., November, 1997 a similar pulsed systems was described. While very few details were described, it appears that mouse infectivity assay was used and with a claimed 100% "inactivation" of Cryptosporidium to a level of 6 logs at energy levels of approximately 200 mJ/cm.sup.2 and greater. The paper claims that the pulsed UV overcomes the "DNA repair mechanism"; however, the UV doses applied are much larger than required with a steady-state medium pressure Hg lamp.
Accordingly, it is an object of the invention to provide a method to treat water in an effective way to so that Cryptosporidium oocysts cannot infect. It is another object of the invention to provide a method using ultraviolet light to render the crysptosporiduium oocysts ineffective to infect. It is yet another object of the present invention to provide a method using ultraviolet light that is cost effective in treating drinking water to eliminate the potential for crysptosporiduium oocysts infection.